<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box{
      position: absolute;
      left: 0;
      right: 0;
      top: 0;
      bottom: 0;
      background-color: #eee;
      display: flex;
      justify-content: center;
      align-items: center;
    }
    .inner{
      position: absolute;
      width: 100px;
      height: 100px;
      /* left: 50%;
      top: 50%;
      transform: translate(-50%,-50%); */
      background-color: #fff;

    }
  </style>
</head>
<body>
  <div class="box">
    <div class="inner"></div>
  </div>
</body>
</html>